This sandbox doesn't contain any VMs to interact with. You would access the public API using the credentials you're provided in the sandbox with the tools available to you on your own machine/in your own environment.
This module on DevNet site is a good place to start getting familiar with the Umbrella API: https://developer.cisco.com/learning/modules/cisco-umbrella/
They also have a quick start guide if you want to get up and running quickly: https://developer.cisco.com/docs/cloud-security/quick-start/#quick-start
Happy to help! Please mark as helpful/solution if applicable.
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